For example, most part of sulfur oxide (SOx) contained in a flue gas exhausted from a boiler of a power plant and the like is sulfur dioxide (SO2), and a part of sulfur oxide is converted to sulfur trioxide (SO3) by catalyst action of metal oxide carried with coexisting combustion ash and a denitration catalyst. In the related arts, since SO3 has high reactivity and corrosivity, neutralization treatment is generally applied by injecting ammonia into a flue gas duct in order to prevent facility deterioration.
When sulfur (S) content originated from fuel in a thermal power plant is subjected to combustion in a boiler, sulfur is oxidized with a catalyst component in the fuel and a catalyst of an NOx removal unit, and then converted from a sulfur dioxide (SO2) to a sulfur trioxide (SO3) component. The converted SO3 has concentration of about 30 to 50 ppm at maximum in a flue gas of a coal burning boiler, and may reach 180 ppm or so at maximum in the case of oil burn/heavy fuel depending on a contamination state in a furnace of the boiler and combustion conditions.
SO3 in the flue gas causes troubles such as corrosion of an apparatus and blockage due to increased adherence of ash in a low-temperature facility located in a downstream of an air heater, and becomes a factor to increase maintenance cost of a power plant user.
Further, SO3 is known as a typical component that causes blue smoke from a stack, and in the case where the blue smoke is highly dense, the plant may be forced to stop operation.
Considering above, in the related arts, some technologies are proposed as measures to remove this SO3, besides introducing ammonia; for example, SOx is removed by spraying CaCO3, Ca(OH)2, CaO, etc., or spraying active carbon ((AC): porous impurity absorption material essentially containing carbon) into the flue gas duct (Patent Literatures 1 and 2).